This invention relates to variable electromagnetic apparatus and more particularly to transformers having means for varying the number of effective turns of the transformer winding or windings.
The usual variable transformer in the prior art has a brush mounted for movement along the length of a winding in the transformer to contact exposed segments of each turn of the winding. By selectively positioning the brush along the winding and by connecting that brush to an output circuit, the effective number of turns in the transformer can be varied, thereby varying the potential at the output. A portion of such a prior art variable transformer is schematically illustrated at 30 in FIG. 1. A core 32 of magnetically permeable material, such as laminated transformer iron, a portion of which is shown, defines at least one loop for magnetic flux to follow. A portion of a winding 34 is shown with turns encircling the core 32. This winding is an output or secondary winding of the transformer, and it may also serve as the input or primary winding, depending on the type of transformer. Changing magnetic flux in the core produces an electromotive force (emf), an electric potential difference, between each turn. The potential differences between turns is cumulative along the length of the winding.
For purposes of clarity, the winding 34 is shown as a line drawing, while the actual cross-section of each turn of the winding is only shown to the right of FIG. 1 at 36. The winding 34 is typically of enamel or varnish insulated copper wire having a square (or rectangular) cross-section as shown. In order to vary the output voltage from the transformer 30, a conductive brush 38 is mounted for movement along the length of the winding, i.e. along a traverse path, so as to contact exposed segments 36 of each turn of the winding. The output voltage is thus dependent on the position of the brush 38 in its traverse path along the winding.
In some positions of the brush in FIG. 1 it may be in contact with only a single winding, depending upon the relative size of the brush as compared to the winding segments 36. However, as shown, the brush may be positioned between two adjacent turns of the winding and thus form an electrical bridge between two turns (and in some prior art variable transformers three turns may become bridged). The exposed segments 36-1 and 36-2 of these adjacent turns, which would otherwise be at different potentials, are thus short circuited by the brush 38. With such a short-circuiting condition a brush of high electrical conductivity would permit a dangerously high current to flow in the turn of the winding defined between the short-circuited segments 36-1 and 36-2. This exceptionally high current would result in excessive heating of the closed winding turn or conductor loop, causing the transformer to self-destruct.
Two prior art techniques are conventionally simultaneously employed to avoid self-destruction of a variable transformer when a brush is positioned in a bridging relationship with two (or more) adjacent turns. First, rather than using a high-conductivity brush, a carbon brush 38 having a relatively high specific resistivity is generally used. The electrical resistance of such carbon brushes serves to limit the current flowing through the closed loop of the bridged turns. Although the so-called "short-circuit current" is thus limited, the high resistance of the carbon brush itself results in electrical losses as well as heating of the brush. The necessity for dissipating the heat generated in the carbon brushes leads to practical problems in designing prior art transformers with high power outputs. Consequently, generally there is a practical upper limit in the size of commercially available variable transformers, namely about 10 KVA per transformer unit. Where larger electrical loads are to be energized, a number of such relatively small prior art units are used simultaneously, and these units are spaced to permit dissipation of heat.
The use of such numerous relatively small prior art variable transformer units for supplying large electrical loads is cumbersome, expensive and wasteful of materials and space, but that is the conventional prior art arrangement.
In addition to the use of carbon brushes, past variable transformers have been manufactured with loose magnetic coupling between the winding and core, that is, the winding is relatively widely spaced from the magnetically permeable core. This loose coupling results in leakage flux illustrated at 40 in FIG. 1, which produces leakage reactance. The leakage reactance, along with the resistance of the carbon brush, serves to limit current flow through a short-circuited turn or turns. The reactances of the many loosely coupled turns have an overall deleterious cumulative effect on the output of the prior art transformer, resulting in undesirably poor regulation of the transformer output voltage.
Moreover, the amount of accumulated leakage reactance in such a prior art variable transformer depends upon the position of the brush along its traverse path. Consequently, the poor voltage regulation characteristics of such a transformer change as the brush is adjusted in its position leading to problems in use.
Furthermore, the relatively large leakage reactances of prior art variable transformers cause electrical losses and result in inefficient utilization of the materials from which the transformer is constructed, mainly transformer iron and copper.
It is among the objects of this invention to provide a variable transformer which provides for variable output by varying the effective number of turns in the transformer winding without short circuiting any turns in the winding and without interrupting the output voltage.
Among the many advantages of the present invention are those resulting from the fact that by eliminating the possibility of short-circuiting turns of the winding, brushes of high electrical conductivity may be used, and the winding may be closely coupled to the core, thereby increasing efficiency and improving output voltage regulation.